These are all because size is not always related to strength/performance. Training methods cause different results. Figure out your training goal and move forward in a way that matches those goals. There are plenty of very strong 132-pound powerlifters, small legs on triathletes, and smallish forearms on rock climbers.
Nutrition. Some people have to experiment with low carbs, moderate carbs, higher carbs to see what prompts fat loss. It's also an issue of playing around with lower or higher calories based on the total training weekly volume and intensity. Also, I'd say most people can "gain weight" with no problem. Making sure those are quality gains is a different story.
Structural. As was said, some mobility should work towards fixing it, depending on exactly what's causing the squat problem.
I meant that before I grip the bar I extend my hand in a L shape and align my thumbs with the rough part of the bar before closing my grip. Regarsless, judging by that video I am way too wide on the bench. Ill try moving in a thumbs length on each side and see how it goes
1: That sounds about right. If you want a big bench press you need horrifyingly strong triceps, lats, and Traps/Rear Deltoids. Whenever my triceps get a nice improvement my bench follows.
2: That is pretty individual, I am actually glad for you since you have an advantage when squatting/deadlifting for a max. My quadriceps are very slow twitch, and high reps make them grow personally. I can easily do about 10 reps with 80% of my max if I want to when squatting.
3: going back to #1 forearms are very important in benching. If you have a weak bench you probably also have tiny forearms. Get better at benching, rowing, deadlifting, tricep extensions, and curls.
4: Metabolism. I gain weight slowly on 4500-4600 calories a day, and I maintain at around 4300-4400.
5: That sounds a lot like me. Take your stance out when squatting, you should at the very least be slightly outside of shoulder width to squat. You will also find that squatting is a nightmarish thing to improve on.
Front squats, good mornings, heavy abs/back work, and more direct leg/hip extension work (reverse hypers, pull throughs, arched back extensions, sumo stiff legged deadlifts, glute ham raises, ect) are your friends. So are sumo deadlifts as they lets you train your glutes/quads/hamstrings with less fucked up leverages. I actually hit a 25 pound personal record after stagnating for a very long time after focusing on sumo deadlifts, and the other stuff I mentioned. And don't forget to try to get strong at squatting in general.